There are a number of misconceptions about sizing a backpack. The most common is that a tall person always needs a bigger pack than a short person. While this may be true, it is not necessarily so. Since Backpacks fit on your back, that is the part of you that we need to measure, not your height. In fact there are actually three aspects to sizing your back pack. The first of these issues is pack length.
Torso length
It is best to enlist a friend or family member to help you at this point. It may be possible to get a measurement by yourself but in my experience it is usually not accurate.
Stand with you feet at shoulder width and have your assistant find your 7th vertebrae. This is found by lowering your chin to your neck and having your assistant locate the bump at the back of your neck, just about even with the top of your shoulders.
Next find the top of your hip bones. This in not in front of your body but along your side. If you find your rib cage under your arm, slide down until you meet the top of your hips, this is your “Illiac Crest”. You will feel sort of a platform there that will support the weight of your backpack and its contents. A properly fitted pack will comfortably transfer most of the weight to this point and off of your shoulders.
Now measure the distance between these two points. This is your “Torso Length”. Nearly all packs are sized based on this measurement. Make sure that whatever “capacity” pack you are searching for, that the “torso length” matches yours.
Sizing your harness
The next measurement to consider is hipbelt and shoulder harness size and gender. Some packs are able to adjust the hip belt angle such as the Gregory Women’s Deva 60 Backpack
The difference is that men’s hips are less shapely than women’s so the angle or flare of the hipbelt should be different. Most men’s hips flare at an angle of 14 to 16 degrees. Women have a more tapered shape that is approximately 20 to 22 degrees. Next determine your hip measurement and select the proper size. These sizes are usually listed in a chart provided by most manufacturers. A properly sized hipbelt will cover the front of your hipbones but will have a room to tighten in front. It will form around the top of your hipbones and not slide down when weighted.
Shoulder harnesses are cut also differently for women than for men. Once again some manuacturers offer adjustable shoulder harnesses and others require changing the harness out for one of the proper dimensions or selecting a pack that is gender and size specific.
A man’s shoulder harness usually has wider straps and extends straight down from the shoulder to the adjusting strap. A womans harness has narrow straps that are “S” shaped to accomodate her shape and is shorter since women are normally shorter from waist to shoulder. Whichever you are looking for, take a close look at the edges of the straps when the pack is on you back and weighted. They should sit flat against your shoulders equally on either side. If you can slip your finger under one side and the other is tight against you, it may cause pain and pressure on that side after some time on the trail.
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